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Why Some AM Radio Stations Don’t Work at Night

Why Some AM Radio Stations Don’t Work at Night

This video was made possible by Learn how to think with Brilliant for
20% off by being one of the first 200 people to sign up over at For all you hip millennial AM radio listeners,
you may have noticed something. A lot of AM radio stations shut down at night
and a lot of other ones can be heard from further away and there’s a good reason for
that. AM radio waves travel further at night. Now here’s some science talk. You might actually have to learn something
this episode. Broadcast radio is essentially split into
two types: AM and FM. AM radio waves look like this, FM waves look
like this. Except this isn’t to scale, this is. You see, AM radio waves are hugely longer
than FM waves. The wavelength from a station broadcasting
at 1000 AM, for example, is 300 meters long while a station broadcasting at 100 FM only
has a wave 3 meters long but with wavelengths, much like with stab wounds, smaller is better,
at least in some ways. You see, the station 1000 AM broadcasts at
1000 kHz which translates to 1,000,000 hertz which means that this wave oscillates 1 million
times per second but the station 100 FM broadcasts at 100 mHZ which translates to 100,000,000
oscillations per second. I won’t go too deep into the science of
this because I want views, but essentially at the higher frequencies FM radio transmits
in a way that is higher quality. So that brings us to nighttime. Between 40 and 600 miles of altitude, there’s
a layer of the atmosphere called the ionosphere. In this region, all the radiation coming from
the sun gives off electrons that stick around for a bit and bounce lower-frequency radio
waves, including AM signals, back to earth, but FM radio waves just go right through. Without the bounce from the ionosphere, FM
radio can only be picked up within line-of-sight of the broadcast tower which, since the earth
supposedly curves, is only about 30-40 miles. At night though, the sun disappears, but you
probably already knew that. Without the sun there’s less radiation giving
off electrons so some of the ionosphere disappears, but not all of it. Above 100 miles the ionosphere remains and
it keeps bouncing the radio signals back to earth, but since the bounce happens higher
up, the signal gets back down to earth further away. This is why AM radio has a significantly longer
range at night. The difference is so significant that an AM
radio station in New York with a transmission distance of about a hundred miles during the
day could, in the right conditions, potentially be picked up by a skilled operator in Europe
at night. But this presents a bit of a problem. There are only 116 AM radio frequencies in
the US as decided by the much loved FCC starting at 540 kHz and increasing in increments of
10 to 1700 kHz but there are thousands of AM radio stations. During the day this isn’t a problem because
their signals only reach a hundred or so miles, but if they all broadcasted during the night
the frequencies would just be a mumbled jumbled mess of sound, so they don’t. Certain frequencies are given clear-channel
status which allow some large stations to broadcast their signal at full power all the
time, but those transmitting on the same frequency during the day are required to shut down or
reduce power overnight. For example, on 1000 AM in the US, there are
two clear channel stations—KOMO in Seattle, Washington and KMVP in Chicago, Illinois. At night, all the nearest stations, like WCCD
in Parma, Ohio, will shut down while those slightly further, like KSOO in Sioux Falls,
South Dakota, will reduce their power significantly so they can only be heard in the immediate
area. This leads to a weird situation where the
broadcasting hours of a station change depending on the the time of year. When these daytime only stations can broadcast
is directly tied to when the sun is up so WCCD, the daytime station in Parma, Ohio,
for example, is allowed to broadcast for 15 hours a day in the summer when days are long
but in the winter, when the sun rises later and sets earlier, they only have 9 hours of
daylight during which they can be on air, but between the clear channel stations, KOMO
in Seattle and KMVP in Chicago, the range is so significant that at night, anyone from
coast to coast can tune into 1000 am and listen to radio. If you want a surefire way to make your millions,
you should start a radio station but if you want to start a radio station you’ll probably
need a better understanding of how radio waves work and there’s no better place to gain
an understanding of scientific concepts than As I’ve been learning in their solar radiation
course, that same radiation that bounces AM radio back to earth is also one of the abundant
and renewable sources of power. Shockingly, you’ll learn complex concepts
like the Shockley Queisser limit simply with their straight-forward explanations, simple
graphics, and thought-provoking puzzles which help you think like a scientist. By going to, you can get
started for free and then, by being one of the first 200 people to upgrade to the Premium
Subscription, you will get 20% off.

100 comments on “Why Some AM Radio Stations Don’t Work at Night

  1. At night I can get AM stations from NYC…I live in Nova Scotia. I can pick them up just from my regular car antenna.

  2. The actual call sign is W MVP not KMVP
    Also, the line that separates the k’s from the W’s is the Mississippi River. A few exceptions would be WBAP Dallas and KYW Philadelphia.

  3. I thought that there were no AM radio station in the world, because I don't have (and most of my family doesn't too) a AM radio transmitor.

  4. KMVP, Chicago's ESPN radio affiliate, broadcasts from my hometown of Downers Grove!

    I never knew I could have such a jolt of excitement in a video about clear channel AM radio.

  5. Man the plugs in this are overbearing. Good vid though. Hopefully my fellow millenials if not succumbing to their stab wounds learned something.

  6. When you mentioned KMVP in Chicago, didn't you mean WMVP? Because Chicago is east of the Mississippi River, and all call signs east of the river begins with W. I bring this up because I live in Phoenix, Arizona, and we have a radio station with the callsign KMVP.

  7. This is very important for people who work out in the open ocean, as one the ways of sending a distress/safety message is using HF equipment (basically AM). We work out a distance to a coast station, from that we know the frequency to use. For 500NM it’s 8kHz, but if it’s night it’s 4kHz. So it’s important to always remember this, as the last thing you want is for your mayday to bounce miles away from station you wanted to get in contact with.

  8. KMVP Chicago isn't a real thing…? Especially since in Chicago radio stations should have the 'W' call sign in-front, right?

  9. Someone may already have mentioned this but I’ll say it again. There is no KMVP in Chicago. It’s WMVP. With a few grandfathered exceptions U.S. broadcast station call letters start with W east of the Mississippi and K west of the Mississippi.

  10. The NAACP forced the FCC to allow WLIB 1190 in NYC to broadcast 24 hours a day in the late 80's,making it much more difficult for Phillies' games on WCAU to be heard in the greater metropolitan area.Then,the scumbags sold the station.The action also cut off WOWO in Fort Wayne,Indiana from being heard.Then,the white man beat them back by creating internet radio.

  11. "If you want a surefire way to make your millions, you should start a radio station."

    I see that doesn't have the slightest understanding of the economics or radio. I personally know of a 2,000 watt AM station that ran a "nonstop music marathon" for five years because he couldn't sell spots, and has finally gone dark and is for sale. I know of another AM station in a major market that ran (until recently) a 24-hour recording begging for someone, anyone to broker time on their station. I don't see millions being made in either case. Despite what claims.

  12. 30-40 mile range for FM?
    I've picked up FM radio in my car from 60-70 miles away from the tower.
    Apparently 103.5 BOB-FM Austin has a big antenna (like 1200 ft) and the place I got it from that far away is around 150 ft lower in elevation

  13. Thanks for the interesting video. Just a point of interest, while most of the year, FM broadcasts only reach into their coverage area, during the summer months, FM stations can be received over 1000 miles away, caused by sporadic E (SpE for short), this is when low VHF and FM signals bounce off the E layer of the atmosphere.

  14. as soon as i watched this video I went over to my radio and easily got stations from nashville and chicago in new jersey. cool.

  15. I hate to do this. No radio station in Chicago would use a K call sign. Everything west of the Mississippi uses K call signs, everything east uses W call signs. KMVP therefore is for a sports radio station out of Phoenix Arizona. WMVP is the ESPN Radio Chicago.

  16. Whips out my radio and tunes to 1000am
    Actually hears KOMO from down in San Francisco
    "Wow, am radio is pretty cool."

  17. I'm Venezuelan and I live in the central coast. During last month's blackouts I realized my car picked up an AM radio station from New York that I later identified as WCBS 880 and I was really really really puzzled about it, because I didn't understand how in heavens could I listen to the same thing that people in NYC were listening to. Thanks for explaining why, I can rest easier now.

  18. The Chicago station is actually WMVP, not KMVP. Unless it's a legacy station, stations are named K*** West of the Mississippi, and W*** East of the Mississippi. KMVP is in Phoenix. KOA in Denver – a 50kW station at 850 AM – has been heard in 38 States, Canada, and Mexico. I believe I have seen some reports that KOA – under some unique conditions – has been heard in Australia and Scotland.

  19. You could have just said 'longer wavelengths travel further at night' – it's simpler. There's nothing special about AM broadcasting in terms of distance. You could do low frequency FM (it's impractical but not impossible – for nerdy technical reasons) and it would travel just as far as AM at night.

  20. You don't need to be 'a skilled operator' to receive American AM stations in Europe. It needs to be at night (obviously) and around 4am works best – when the incredible radio interference noise from nearby TV's, wall wart PSUs, PCs, induction hobs etc is less. You also need a radio with a 'ext ant' socket to which you can connect a decent bit of high wire. Simple AM/FM radios only use an internal loopstick antenna which works best with local stations.

    American stations come roaring into the UK, especially in winter months. Unless you're particularly concerned to know how Jesus can save you from sin, the usual show content may not be your thing, though…

  21. KMVP is not an AM station in Chicago. It's an AM and FM station in Phoenix, AZ. I believe you meant WMVP.

  22. This explains why I hear chinese propaganda in the radio at night, and some jesus thing about jesus and god that comes probably from where you live. It's making things intresting.

  23. During field day 2019 (ham radio thing) at 2-3ish am I was picking up Chinese/Japanese brodcast stations on West coast Canada and other reported hearing it west coast us as well

  24. @0:26 that's a device to measure air pressure but then again I haven't listened to the science part yet. If I've learned anything from this channel things aren't as I expect

  25. Perhaps too technical for your target audience, but you failed to mention that many AM stations, rather than signing off or drastically reducing power, shift to directional antenna patterns after sunset. I worked in broadcasting for a few years in the early ‘70s. The first place I worked was a small town AM & FM outlet, WCOR in Lebanon, TN. They were 1000 watts on AM and 10,000 watts FM. At sunset, the AM shut down and the FM continued. They simulcast, broadcasting the same content on both transmitters. Later I worked for WLAC in Nashville. Their AM & FM were separate operations. The FM at 105.9 MHz broadcast 24 hours with 100,000 watts ERP. The AM was 50,000 watts at 1510 KHz. Both were the maximum legal power for U.S. stations. At sundown the AM shifted to a directional pattern, oriented north/south if I remember correctly. Another Nashville station, WSM at 650 KHz, broadcast 24 hours with their full licensed 50KW and an omnidirectional pattern. The power and reach of WSM, “clear channel 650,” is one of the reason the Grand Ole Opry, and country music in general became so popular.

    A bit of radio trivia for you. Did you know that WLS 890 KHz in Chicago was started by Sears, Roebuck and Co. to encourage people to buy radio receivers? The call letters were an acronym for Sears slogan, the World’s Largest Store. Mid-1920’s editions of the Sears catalog contain full page ads for WLS opposite the page selling radio sets.

  26. AM frequencies can also bounce off water. So you can get cases where it hops into the atmosphere, off the ocean (or lake), back off the atmosphere, etc. and gets half way around the planet. In the military, we'd occasionally tune in our HF into a local AM station for music. Half-hour later, you could hear a faint signal, re-tune it a few hundred megahertz and you'd be listening to an Iranian station. You could often guess the station by where it was dark at the time of day you are hearing it and eliminating languages you could identify. We had a big chart of world stations which could finally tell you who it was.

  27. It is very oblivious, that ypu do not know about FM smd TV DXing.

    As a retired radio/TV broadcast engineer with 40+ years under my belt, band conditions change quite often…even the 11 year sunspot cycle affects all signals no matter what the transmitted frequency.

  28. If you are doing 'science talk' don't equate broadcast mode with frequency band. I know that's how the bands have long been referred to in the US, and we in the UK eventually adopted it because of imported japanese radios, but it's wrong!

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